Flink: Add decimal write/read roundtrip test for FlinkParquetReaders

[wombatu-kun]

Summary

• Address the longstanding TODO in FlinkParquetReaders.BinaryDecimalReader by adding a Flink-writer → Flink-reader roundtrip test for decimals across precisions 9/2, 15/3 and 38/10 (covering INT32/INT64/FIXED_LEN_BYTE_ARRAY physical encodings).
• Test added to existing TestFlinkParquetReader in v1.20, v2.0 and v2.1; the TODO comment is removed.

[pvary]

Should we test this in the BaseFormatModelTests?

[wombatu-kun]

Good call, thanks @pvary — done in 6d2f777.

Rather than a Flink-only test I added a StructWithDecimals generator (decimal(9,2) / (15,3) / (38,10), covering the Parquet INT32 / INT64 / FIXED_LEN_BYTE_ARRAY encodings) to DataGenerators.ALL, so decimals now flow through the whole BaseFormatModelTests matrix for Flink and Spark across Avro/Parquet/ORC.

One gap I hit while doing this: the generator-parameterized cases were only engineWrite→genericRead and genericWrite→engineRead, so the engine-writer→engine-reader roundtrip the original TODO asked for ("write-read-validate decimal via FlinkParquetWrite/Reader") wasn't actually covered by any single case. I added a testDataWriterEngineWriteEngineRead case to BaseFormatModelTests to close that symmetry — for all types/engines/formats, not just decimals.

The duplicated TestFlinkParquetReader decimal test is removed from v1.20/v2.0/v2.1; the FlinkParquetReaders TODO stays resolved by the broader shared coverage. Verified green: TestFlinkFormatModel (Flink 1.20/2.0/2.1) and TestSparkFormatModel (Spark 4.1).